More precisely, the invention relates to the technical field of high-capacity broadband multibeam satellite networks. As illustrated on FIG. 1A, a multibeam satellite network comprises a plurality of gateway stations (or simply “gateways”) GW1, GW2 . . . GWN, which are usually connected to a terrestrial telecommunication network TN (e.g. the Internet). Bi-directional (or sometimes one-directional) communication is established between said gateway stations and user terminals UT1, UT2, . . . UTM, distributed over a region of interest ROI, through a geostationary multibeam satellite SAT. Said satellite generates two sort of beams: feeder beams FB1 . . . FBN, which are associated to respective gateway stations, and user beams UB1 . . . UBK, which are associated to user terminals; usually, several user terminals are located within the footprint of a single user beam and share its resources using Time and/or Frequency Division Multiple Access, while there is normally a single gateway section per feeder beam.
Some vocabulary has to be introduced at this point:                each (usually bidirectional) connection between a gateway station GW and the satellite SAT is called a feeder link FL;        each (usually bidirectional) connection between the satellite SAT and a user terminal UT is called a user link UL;        a ground-to-satellite connection is called an uplink upL, and a satellite-to-ground connection is called a downlink dwL;        a one-directional gateway station—user terminal connection is called a forward link fwL; it comprises a uplink section belonging to a feeder link and a downlink section belonging to a user link;        a one-directional user terminal—gateway station connection is called a return link rtL; it comprises an uplink section belonging to a user link and a downlink section belonging to a feeder link.        
This is illustrated on FIG. 1B.
In known systems, each gateway station serves a plurality of user beams, but each user beam is served by a single gateway station. Therefore, feeder links need a much larger throughput than user beams.
As the available feeder link bandwidth is limited, it is common practice to fully reuse the frequency placing the gateway stations far enough from each other to ensure spatial isolation from the satellite multi-beam antenna. Typically the feeder link is using a dedicated frequency band separated from the user link one; in certain cases both the feeder link and the user link frequency bands can be reused by means of antenna beams spatial separation. On the contrary, as user terminals are scattered over all the region of interest, adjacent user beams use different frequencies and/or polarizations; indeed, as the user links bandwidth is constrained (e.g. due to regulatory aspects), the capacity of the system can only be boosted increasing the frequency reuse factor thanks to the partitioning of the coverage region among the largest number of beams compatible with space segment capability.
The wanted increase in the number of user beams may find limitations in the feeder links available throughput. This is because the required throughput increases linearly with the number of user links. More precisely, assuming a regular frequency reuse pattern for the user links (UL) characterized by NCUL colours (obtained by means of frequency or polarization reuse) over NBUL user beams with a total user link bandwidth allocation BUL, then the required aggregate feeder link bandwidth is given by:
                              B          Tot          FL                =                                            N              B              UL                        ⁢                                                            B                  UL                                ⁢                                  N                  P                  UL                                                            N                C                UL                                              =                                    N              B              UL                        ⁢                          B              B              UL                                                          (        1        )            where BBUL is the bandwidth per beam and NPUL indicates the number of polarizations exploited in the user link frequency/polarization reuse scheme. It is worth mentioning here that in most of current multibeam systems, the total available band in the user link BUL is segmented in different parts commonly named colours in the polarization and/or frequency domain. A typical arrangement of a frequency plan is shown in the upper part of FIG. 6. As mentioned above, segmentation of the available band is necessary to isolate adjacent beams, which otherwise would interfere with each other jeopardizing the proper operation of the system. As a consequence, the user link band per beam is given by
      B    B    UL    =            B      UL        ·                            N          P          UL                          N          C          UL                    .      
The minimum number of required gateways NGWReq is then given by:
                              N          GW          Req                =                              ⌈                                          B                Tot                FL                                            B                                  G                  .                  W                  .                                FL                                      ⌉                    =                      ⌈                                          N                B                UL                            ·                                                                                          B                      UL                                        ⁢                                          N                      P                      UL                                                                            N                    C                    UL                                                                                                              B                      FL                                        ⁢                                          N                      P                      FL                                                                            N                    C                    FL                                                                        ⌉                                              (        2        )            
where ┌ ┐ represents the largest integer operator, BGWFL is the available feeder link bandwidth per gateway (including, if applicable the use of double polarization), BFL is the frequency bandwidth allocated to the feeder link, NPFL indicates the number of polarizations in the feeder link frequency/polarization reuse scheme and NCFL is the number of “colours” in the regular frequency reuse pattern of the feeder link. Note that it is common that double polarization is used for the feeder links (NPFL=2) and that all gateways share the same frequency thanks to the satellite gateway beams good spatial isolation (NCFL=1).
Most communication systems of the kind depicted on FIG. 1A operate in the Ka-band, using part of this band for the feeder links and another part of it for the user links. To further increase the capacity, it has been proposed to allocate the whole Ka-band to the user links, moving the feeder links to higher frequencies, e.g. the Q/V band (40-50 GHz).
For example, a state-of-the-art Ka-band system may have NBUL=100 user beams using part of the protected Ka-band (i.e. BUL=0.5 GHz) and a Ka-band feeder link band with BFL=1 GHz, 4 colours scheme with polarization reuse in the user links (NCUL=4, NPUL=2) and full frequency reuse with double polarization in the feeder link (NCUL=1, NPUL=2); then, according to equation (2), the required number of gateway stations is NGWReq≧13.
In case of a more aggressive scenario with Ka-band user link and Q/V-band feeder link with NBUL=100 beam system using the shared portion of the Ka-band (i.e. BUL=2.9 GHz) and a Q/V-band feeder link band with BFL=4.5 GHz, 4 colours scheme with polarization reuse in the user link NCUL=4, NPUL=2) and full frequency reuse with double polarization in the feeder link (NCFL=1, NPFL=2), equation (2) gives NGWReq≧17.
The satellite feeder links are key elements of the satellite system availability budget as each gateway station typically serves a certain number of user beams, each supporting several users. Therefore, outage of a single gateway station, e.g. due to rain attenuation, may cause a complete service interruption for a large number of users, which is not acceptable. As a consequence, the feeder links have to be designed to provide a very high link availability, typically in the order of 99.9% or higher. This design approach ensures that, for a given system availability target, the atmospheric fading is mainly affecting the satellite user link. In the feeder forward link, a typical Fade Mitigation Techniques (FMT) is the uplink power control, providing a few more dBs of extra margin at the expenses of a bulkier gateway RF front-end. For the user link, Adaptive Coding and Modulation (ACM) is nowadays a common fade mitigation technique which has been adopted by modern satellite broadband standards for the forward and the return link such as DVB-S2 and DVB-RCS2. However, these techniques might not be sufficient to provide the required availability level of the feeder links, particularly when the Q/V-band is used, as at these very high frequency rain phenomena can create severe link impairments (i.e. atmospheric attenuation) that, depending on the rain intensity and satellite elevation angle, can exceed 15-20 dB for a non negligible percentage of time during the year.
The “site diversity” concept is a well known strategy to mitigate the impact of fading events in the satellite feeder links ([1], [2]). As represented on FIG. 2A, in this approach, every active (or “nominal”) gateway station GW1, GW2 is complemented by a backup gateway station GWB1, GWB2. In normal conditions, the backup gateway stations are idle; however, as illustrated on FIG. 2B, when a nominal gateway station (e.g. GW1) undergoes an unacceptable level of fading (e.g. due to heavy rain) or a failure, then the corresponding backup gateway station (GWB1) is activated to replace it. The backup gateway station is usually located at a distance of several tens of kilometres from the nominal gateway location, greater than the size of a typical heavy rain cell, but within the same feeder beam. In this way the payload is fully transparent to this technique and does not need any specific modification compared to a traditional repeater. On the ground segment side, every pair of nominal—backup gateway stations needs to be connected through a high capacity network to re-route the traffic whenever a gateway switch occurs. This approach is quite practical for satellite networks characterized by one or few beams as a second gateway greatly improves the availability at an acceptable ground cost increase. Instead, for high capacity multi-beam satellite systems requiring a large number of gateways to support the high feeder link throughput, site diversity turns out to be very—and possibly unacceptably—costly in terms of the ground segment investment, as the already relevant number of gateway stations has to be doubled with respect to a diversity-less implementation.
An alternative approach, called “gateway soft diversity”, was introduced in [3]. In this concept, a number of gateway stations (geographically separated but all located within the same feeder link beam) serve at the same time a certain amount of user beams, the sharing being implemented by means of a Frequency Division Multiple Access (FDMA) arrangement instead of the conventional full site diversity. In case of uplink fading causing one gateway to be in outage, the users assigned to the carriers served by the gateway in outage are switched by a central controller to the carriers managed by another gateway serving the same beam. As key advantages compared to the conventional approach, reference [3] mentions the fact in case of two gateways in diversity the gateway EIRP may be halved as each gateway has to handle half of the carriers with the same availability (but 50% feeder link bandwidth reduction). Furthermore assuming 4 carriers for the user downlink each soft diversity gateway will uplink only two carriers so there is about 2 dB reduction in the required gateway HPA OBO (however this OBO advantage will not be present when there gateway handles more than 4 carriers as it typically the case).
The “soft diversity” concept proposed in [3] assumes that the user terminal has frequency agility in terms of being able to tune on different downlink carriers according to instructions received by the radio resource management device. This capability is available on standard user terminals.
As disclosed by reference [3], “soft diversity” is a single feeder beam concept; its generalization to the case of multiple feeder beams is not straightforward. In any case, such a generalization could not provide a solution to the cost issue discussed above, as two or more gateway station would be required for each feeder beam. In fact, reference [3] does not allow redistributing the traffic among the other feeders in case one (or more) feeder links would become unavailable e.g. due to an intense rain event.
It should also be noted that, despite the gateway redundancy it introduces, the “soft diversity” concept cannot avoid a throughput reduction in case of fading affecting a gateway station, which is not the case of the conventional “site diversity” concept.
Reference [4] discloses two different approaches to mitigate the impact of fading events in the satellite feeder links at a lower cost than the conventional “site diversity” concept described above.
The first approach of reference [4] also exploits site diversity. However, diversity is not done on a one-to-one basis as in the conventional approach. Instead, the communication system comprises a first number of “nominal” gateway stations, and a second number of “diversity” gateway stations, situated outside the region of interest and shared among the multiple satellite feeder links. When a “nominal” gateway station is in outage, the corresponding data traffic is switched to an available “diversity” gateway. For example, in a case where NGWReq=6, two “diversity” gateway stations may be used, leading to a total of eight gateway stations instead of twelve as required by the conventional “site diversity” approach.
In the second approach of reference [4] there is no spatial diversity. However, all the gateway stations use only part of the available bandwidth, leaving a set of unused “diversity channels”. If one or more gateways stations are in outage, their data traffic is dynamically allocated to the available “diversity channels” of the remaining gateway stations. Like the “soft diversity” approach of reference [3], this concept requires frequency-agile user terminals; however, fading does not necessarily results in a reduction of the throughput.
The two approaches of reference [4] can also be combined.